1. Field of the Invention
The present invention relates to a thermosensitive color printing method and a thermosensitive color printer for printing a full-color image on thermosensitive color recording paper in a frame sequential fashion. More particularly, the present invention relates to a thermosensitive color printing method and a thermosensitive color printer which smooth the surface of the thermosensitive color recording paper after having an image recorded thereon.
2. Background Arts
In a thermosensitive color printer, thermosensitive color recording paper, hereinafter called simply the recording paper, is directly heated by a thermal head that is pressed onto the recording paper while the recording paper is transported. As the recording paper is heated, color dots are developed on the recording paper.
As shown in FIG. 14, the recording paper 10 has a thermosensitive cyan coloring layer 12, a thermosensitive magenta coloring layer 13, and a thermosensitive yellow coloring layer 14 formed atop another on one side of a base material 11. A transparent protective layer 15 is formed atop the thermosensitive coloring layers 12 to 14, for protecting the coloring layer 12 to 14 from scratches or stains. The sequence of forming these three coloring layers 12 to 14 is not limited to that shown in the drawings, and the three coloring layers 12 to 14 have different heat-sensitivities from each other that decrease with the depth or distance of the respective layers from an obverse surface 10a of the recording layer 10. Intermediate layers 16 and 17 are formed between these three coloring layers 12 to 14, for adjusting the heat-sensitivities of the respective coloring layers 12 to 14. A back protective layer 18 is formed on the opposite side of the base material 11 from the obverse protective layer 15.
In the recording paper 10 shown in FIG. 14, the cyan coloring layer 12 has the lowest heat-sensitivity and the yellow coloring layer 14 has the highest heat-sensitivity . Accordingly, as shown in FIG. 15, the yellow coloring layer 14 needs the smallest heat energy to develop yellow color, whereas the cyan coloring layer 12 needs the largest heat energy to develop cyan color. Because of the difference in heat-sensitivity between the three coloring layers 12 to 14, it is possible to record three color frames sequentially from the highest sensitive coloring layer to the lower sensitive coloring layer by applying increasing amounts of heat energy to the recording paper 10 from one color after another.
To stop the coloring layer from being developed unnecessarily by the heat energy applied for recording the next color frame, the coloring layer having a color frame recorded thereon is fixed by electromagnetic rays of a specific range before the next color frame is recorded. In the recording paper 10, the magenta coloring layer 13 has an absorption spectrum whose peak wavelength is at about 365 nm, and loses coloring ability when it is exposed to ultraviolet rays of this wavelength range. On the other hand, the yellow coloring layer 14 has an absorption spectrum whose peak wavelength is at about 420 nm, and loses coloring ability when it is exposed to violet visible light of this wavelength range. So the violet visible light of 420 nm is projected onto the recording paper 10 after the yellow frame is recorded, before the magenta frame is recorded. After recording the magenta frame, the ultraviolet rays of 365 nm is projected onto the recording paper 10 to fix the magenta coloring layer 13.
The protective layer 15 is made from a transparent heat resistant resin material. As well-known in the art, the heat resistant resin material starts to be softened above a glass transit point or temperature of the main component of the resin material. The glass transit point is different between different resin materials. For example, in a conventional thermosensitive color recording paper, PVA (poly-vinyl-alcohol) is used as the main component of the protective layer, whose glass transit temperature is about 70xc2x0 C.
The thermal head has an array of glaze layers formed on a substrate, and a heating element is located around a peak of a semi-cylindrical glaze layer whose axis extends perpendicularly to the paper transporting direction. The heat energy applied to the recording paper for developing colors is so high that the protective layer is softened and the temperature of the protective layer of the heated portion can be still above its glass transit point even after the heated portion is removed from the glaze layer. In that case, additives contained in the protective layer, such as an anti-blocking agent, emerge to the obverse surface of the recording paper, providing irregular fine roughness on the obverse surface, that lessens the glossiness of the obverse surface, and coarsens the printed image.
To restore the glossiness of the recording paper after having an image recorded thereon, according to a conventional smoothing process, a flat smooth sheet is laid over the recording paper, and the recording paper is squeezed together with the flat smooth sheet through a pair of heating rollers, thereby to hot-press the recording paper. However, this conventional smoothing process needs a specific smoothing apparatus in addition to the printer, and the flat smooth sheet must be laid over the recording paper by hands. Moreover, it has been difficult to maintain the amount of heat energy applied from the heat rollers to the recording paper in a range suitable for smoothing.
To solve this problem, a smoothing method has been suggested in JPA 10-291332, wherein a second thermal head for smoothing is provided in addition to a thermal head for recording, so as to heat the recording paper uniformly by the second thermal head after three color frames are sequentially recorded by the first thermal head. This prior art also discloses a teaching to use the same thermal head for recording and smoothing.
However, optimum contacting conditions of the heating elements with the protective layer for smoothing are different from those optimum for recording. Where the contacting conditions of the heating elements are optimized for smoothing, printing quality or heating efficiency would be lowered. Where the contacting conditions of the heating elements are optimized for recording, the effect of smoothing would be insufficient. Especially, the smoothing effect increases with an increase in pressure from the heating elements to the recording paper, but large pressure on the recording paper would cause fluctuation in transport speed of the recording paper, and thus color failures between the tree color frames of one full-color image. Besides that, the larger the pressure of the heating elements on the recording paper, the sooner the thermal head will be worn out. Therefore, it has been difficult to achieve both adequate coloring quality and highest glossiness by using the same thermal head for recording and smoothing.
In view of the foregoing, an object of the present invention is to provide a color thermosensitive printing method and a thermosensitive color printer for printing a full-color image in a frame sequential fashion on thermosensitive color recording paper having a plurality of coloring layers formed on atop another and a heat resistant protective layer formed on an obverse surface, which method and printer can smooth the surface of the thermosensitive color recording paper adequately by use of the same thermal head as used for recording, while maintaining good coloring quality.
To achieve the above object, a thermosensitive color printing method of the present invention comprises the steps of:
A. recording different color frames of the full-color image line by line on the respective coloring layers sequentially from the obverse side by pressing an array of heating elements of a thermal head onto the obverse surface of the thermosensitive color recording paper and heating the recording paper by the heating elements while supporting the thermosensitive color recording paper from a reverse side by a platen member and transporting the recording paper through between the thermal head and the platen roller;
B. fixing an upper one of the coloring layers optically before recording on the next coloring layer by projecting rays of a specific wavelength range onto the thermosensitive recording paper;
C. transporting the thermosensitive color recording paper, after having the full-color image recorded thereon, while pressing the heating elements onto the obverse surface of the recording paper with a higher pressure than during the step A; and
D. heating the thermosensitive color recording paper, during the step C, by the heating elements to an extent predetermined for smoothing the protective layer.
According to another aspect of the present invention, a thermosensitive color printing method of printing a full-color image in a frame sequential fashion on the thermosensitive color recording paper comprises the steps of:
A. recording different color frames of the full-color image line by line on the respective coloring layers sequentially from the obverse side by pressing an array of eating elements of a thermal head onto the obverse surface of the thermosensitive color recording paper and heating the recording paper by the heating elements while supporting the thermosensitive color recording paper from a reverse side by a platen roller and transporting the recording paper back and force along a paper transport path that extend perpendicularly to the array of heating elements;
B. fixing one color frame optically before recording the next color frame by projecting rays of a specific wavelength range onto the thermosensitive recording paper;
C. transporting the thermosensitive color recording paper, after having the full-color image recorded thereon, in one direction along the transport path while pressing the heating elements onto the obverse surface of the recording paper at a position that is shifted from a contact position of the heating elements in the step A relative to the platen roller, to an upstream side with respect to the paper transporting direction in the step C; and
D. heating the thermosensitive color recording paper, during the step D, by the heating elements to an extent predetermined for smoothing the protective layer.
It is preferable to press the heating elements onto the obverse surface of the thermosensitive color recording paper with a higher pressure for smoothing than for recording, besides shifting the contact position of the heating elements for smoothing from the contact position for recording.
In a thermosensitive color printer for printing a full-color image on thermosensitive color recording paper having a plurality of coloring layers formed on atop another and a heat resistant protective layer formed on an obverse surface of the thermosensitive color recording paper, the coloring layers having decreasing heat-sensitivities from the obverse side to develop different colors from each other, wherein different color frames of the full-color image are recorded on the respective coloring layers sequentially from the obverse side by heating the recording paper and then fixing one color frame optically before recording the next color frame, the present invention is characterized by comprising:
a thermal head having an array of heating elements, the heating elements being pressed onto the obverse surface of the thermosensitive color recording paper to heat the recording paper;
a platen roller opposed to the array of heating elements, for supporting the recording paper from a reverse side;
a transporting device for transporting the thermosensitive color recording paper along a paper transport path that extends perpendicularly to the array of the heating elements;
a driving device for driving the heating elements to heat the thermosensitive color recording paper as it is transported along the paper transport path, for recording the full-color image and thereafter for smoothing the protective layer;
an optical fixing device for projecting optical fixing rays onto the thermosensitive recording paper;
a pressure changing device for changing pressure from the heating elements to the recording paper between a lower value for recording and a higher value for smoothing; and
a contact position shifting device for shifting the position of the heating elements pressed on the recording paper between a first contact position and a second contact position that is located at an upstream side of the first position in the paper transporting direction during the smoothing.